Duplicating stencils



April 1968 A. F. BLAKE ETAL 7 3,376,810

DUPLICATING STENCILS Filed Oct. 21, 1966 Stenci/ zf/ssue fm k pregnated w/h imp erv/oas moter/a/ rendered conductive by inc/us/on of carbon b/oc'k.

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United States Patent 3,376,810 DUPLICATING STENCILS Alan Francis Blake, London, and Cyril Green, Barnet,

England, assignors to Gestetner Limited, London, England, a British company Continuation-impart of application Ser. No. 388,550, Aug. 10, 1964. This application Oct. 21, 1966, Ser. No. 588,426 Claims priority, application Great Britain, June 21, 1963, 24,890/ 63 7 Claims. (Cl. 101-128.2)

ABSTRACT OF THE DISCLOSURE The invention provides a duplicating stencil, designed to be cut electrically, made of a sheet of thin light stencil tissue strengthened with a modified polyamide resin and impregnated with ink-impervious material containing electrically conductive particles, the said sheet being attached to a backing sheet having a relatively highly electrically conductive layer coated thereon.

This application is a continuation-in-part of our application Ser. No. 388,550, filed Aug. 10, 1964, now abandoned.

This invention relates to duplicating stencils which can be out both mechanically (as with a typewriter) and electronically.

The conventional duplicating stencil used in enormous numbers all over the world consists of a thin sheet of highly porous tissue impregnated with an ink-impervious material. (The impregnated sheet of tissue is called the stencil sheet) When the stencil is cut, the ink-impervious material is displaced, e.g., by the impact of a typewriter key, the fibres of the porous tissue are exposed, and, since the tissue itself offers little or no resistance to the passage of ink, the stencil will then pass ink in the cut areas. The highly porous tissue in this type of duplicating stencil is known in the art as stencil tissue. It is quite different from ordinary impervious thin papers since it is made from relatively long fibres, generally unbeaten abaca fibres in Europe and the United States and kozo (also called yoshino) fibres in Japan, and contains at least about of void-area, whereas impervious thin papers are made from short fibres (e.g., fairly well beaten, flax fibre) and contain very low void-areas. Thus, the thin paper used in the duplicating stencil described in Patent No. 3,151,548, of common assignee with the present application, has a porosity corresponding to less than 0.01% of void-area. Such impervious paper, however thin, cannot pass ink until it has actually been perforated, e.g., with a spark or mechanically with a sharp instrument such as a wheel pen.

In order to make it possible to cut stencils made with stencil tissue electronically as well as mechanically, it has been proposed to render the ink-impervious material electrically conductive by incorporating in it electrically conductive particles.

A known stencil of this kind (see French Patent No. 1,283,204) comprises a stencil sheet of stencil tissue impregnated with an ink-impervious material rendered conductive by inclusion of carbon or other conductive particles, the stencil sheet being capable of being cut me- Patented Apr. 9, 1968 chanically, as with a typewriter, a backing sheet attached to the stencil sheet along one edge, and a layer of relatively high electrical conductivity located between the stencil sheet and the backing sheet. This relatively highly conductive layer is generally coated on the backing sheet but it can also be carried on a separate sheet between the two.

The present invention provides an improved form of stencil of this general kind in which a novel kind of stencil sheet is used. As compared with the known form of stencil, the stencil of this invention is less likely to give rise to set-off during duplicating because of employment of a stencil tissue lighter in weight, more porous and thinner than that generally used in the manufacture of pressure-sensitive stencils, including that used in the French patent quoted above. It can also be more sharply cut electrically (so as to give clearer duplicated copies). In a preferred form, the stencil of the invention can be made so that the void-area produced by the application of a given voltage is linearly related to that voltage within wide limits, so that it is possible to use the new stencils to obtain truer results in the facsimile reproduction of continuous tone originals.

The duplicating stencils of this invention comprise a stencil sheet made of stencil tissue weighing less than 8 grams per square metre and less than 0.0015 inch thick, i.e. thus lighter and thinner (and also more porous) than conventional stencil tissue, strengthened with a modified polyamide resin (as hereinafter defined) and optionally incorporating a highly conductive carbon, the said tissue being impregnated with an ink-impervious material, including electrically conductive particles, e.g., of carbon black, but of relatively low electrical conductivity, the said stencil sheet being capable of being cut mechanically, a backing sheet attached to the stencil sheet along one edge and a relatively highly electrically conductive layer coated on the backing sheet and located between the stencil sheet and the backing sheet. The modified polyamide resin used for strengthening the tissue is a product formed by condensing a polyamide with an epihalohydrin, e.g., epichlorohydrin, so that the secondary amido groups react with formation of an azidine ring (see US. Patent No. 2,926,154). A preferred such modified polyamide is that sold under the trade name Kymene 557 by the Hercules Powder Company. When it is desired that the stencil sheet should incorporate a highly conductive carbon, the incorporation of the latter may be efifected in the manner described below.

Conventional stencil tissue is of the order of 9-13 grams per square metre and 00018-00025 inch thick. As already stated, it has a void-area of at least about 20%. Attempts to make thinner paper by calendering have led to a loss of porosity with consequent loss of sensitivity of the coated stencil and introduction of the disadvantage of cut-out. If, on the other hand, the weight of the paper is reduced to 6-7 grams per square metre, the paper becomes too weak and tearing of the stencil in use could occur. Whilst the strength of the paper can be improved with any recognized paper-strengthening resin, such as melamineor urea-formaldehyde resin, it has been found that modified polyamides increase the strength of the tissue both in the dry and wet state with a minimum of interference with both the electric cutting properties of the paper and its cut-out resistance when typed on in the traditional manner. The stencil tissue used in this invention weighs preferably 6-7 grams per square metre, is preferably 0.0013-0.0014 inch thick and is strengthened by impregnation with a modified polyamide resin, e.g., that sold as Kymene 557 by the Hercules Powder Company. Its porosity is greater than that of conventional stencil tissue and corresponds to a void-area of at least about 30%.

As already stated, the stencil tissue may also incorporate a conductive carbon. By treating the surface of the fibres of the stencil tissue so as to bind to them particles of highly conductive carbon black, the electrical resistance of the stencil tissue may be reduced to the same order as that of the ink-impervious material with which the said tissue is impregnated and in consequence a sharper final duplicated copy is obtained because of the elimination of interference by the non-conductive cellulose fibres. The conductive carbon, e.g., that sold under the Mark XC 72 by Cabot Carbon, is incorporated in the stencil tissue after manufacture, e.g., by treating the tissue in a size press with a slurry of the carbon in dilute carboxymethyl cellulose solution (or a solution of similar viscosity) containing to of the above-mentioned carbon, the preferred amount being about 7%.

The ink-impervious material in the stencil tissue is applied in the conventional way by impregnating with a solution of nitro-cellulose composition plasticized to render it capable of being cut on a typewriter and incorporating electrically conductive particles to render it electrically conductive. The resistance of the stencil sheet measured using electrodes of 1 sq. cm., 1 cm. apart under a load of 2 kg., is preferably 5,000-l00,000 ohms, the preferred figure being about 30,000 ohms. The electrically conductive particles incorporated in the ink-impervious material may be of a highly conductive carbon, e.g., Vulcan XXX Black, or metallic particles, in amount preferably about 10% by weight. This is perfectly satisfactory if the stencils are to be used for line work but, as this formula does not result in a linear voltage-void relationship, it is not satisfactory if the stencil is to be used to produce continuous tone originals. In the latter circumstances, a carbon of lower conductivity, e.g. that sold under the designation Elf 8 by Cabot Carbon, should be used but in higher proportion, preferably -30% by weight so that the electrical resistance is of the same order as obtained with the highly conductive carbon. When this is done, the stencil obtained can be punctured by the passage of an electric spark so that the void-area produced is proportional to the voltage applied and reproduction of continuous tone originals is possible. In general, it can be said that highly conductive carbon particles have an electric resistivity of 0.03 to 0.1 ohm-inch at an apparent density of 44 lbs. per cubic foot while particles of relatively low conductivity have a resistivity greater than 2 ohm-inches at the same density.

The mounting of the stencil sheet on the backing sheet and the coating of the highly conductive layer on the backing sheet is carried out in conventional manner. The highly conductive layer may be obtained by roller coating onto the backing sheet a solution of a plasticised polyvinyl resin, e.g., polyvinyl chloride, containing highly conductive carbon black. The electrical resistance of this layer measured in the manner described above will ordinarily be 1,000-2,000 ohms.

The following example describes the production of a preferred stencil in accordance with the invention.

EXAMPLE A sheet of stencil tissue weighing 6-7 grams per square metre and 0.0013-0.0014 inch thick is wet strengthened with Kymene 557 (Hercules Powder Company) and then 4 optionally treated on a size press with a slurry containing 7% carbon black (XC 72 Cabot).

The sheet is then impregnated to a weight (dry) of 45 grams per square metre with a nitrocellulose solution containing:

, Grams Nitrocellulose HM 15/20 (I.C.I.) 53 Tritolyl phosphate 21 Diethylene glycol 18 Butyl stearate l8 Oleic acid 320 Carbon black (Elf 8 Cabot) In solvent:

Ethyl acetate 260 Industrial methylated spirit 800 The resistance of this stencil sheet is 30,000 ohms measured in the manner described above.

This stencil sheet is mounted on a backing sheet, weighing 60-90 grams per square metre, coated to a coating weight (dry) of 6 grams per square metre, with a P.V.C. solution containing:

Grams P.V.C. copolymer VYHH (Bakelite) 30 Tritolyl phosphate 10 Acetylene Black (Shawinigan) 18 Solvent: Methyl ethyl ketone This coating has a resistance measured in the manner described above of 1,000-2,000 ohms.

A partial cross-section of a duplicating stencil in accordance with this invention is shown diagrammatically in FIGURE 1 of the accompanying drawings which is self-explanatory. The thickness of the two sheets and of the layer on the backing sheet as well as the size of the kozo fibres have been much exaggerated for purposes of clarity.

We claim:

1. A duplicating stencil comprising a stencil sheet made of stencil tissue weighing less than 8 grams per square meter, being less than 0.0015 inch thick, having a void area of at least about 30%, and being strengthened by a modified polyamide resin, said tissue being impregnated with an ink-impervious material including electrically conductive particles, but of relatively low electrical conductivity, the resistance of said stencil sheet being in the range of 5,000-100,000 ohms, said stencil sheet being capable of being cut mechanically, a backing sheet attached to the stencil sheet along one edge and a relatively highly electrically conductive layer coated on the backing sheet and located between the stencil sheet and the backing sheet, the resistance of the highly conductive layer being in the range of 1,000 to 2,000 ohms, said resistances being measured using electrodes of 1 sq. cm., 1 cm. apart under a load of 2 kg.

2. A stencil according to claim 1 in which the tissue incorporates a highly conductive carbon in addition to the electrically conductive particles included in the ink-impervious material.

3. A stencil according to claim 1 in which the stencil tissue weighs 6-7 grams per square metre and is 0.0013 to 0.0.014 inch thick.

4. A stencil according to claim 1 in which the inkimpervious material used to impregnate the said stencil tissue contains about 10% by weight of highly conductive carbon or metallic particles.

5. A stencil according to claim 1 in which the inkimpervious material used to impregnate the said stencil tissue contains 20-30% by weight of carbon particles of relatively low conductivity.

6. A stencil according to claim 2 in which the stencil tissue having carbon incorporated therein has approxi- 5 mately the same electrical conductivity as the ink-impervious material including electrically conductive particles used to impregnate the said tissue.

7. A stencil according to claim 1 in which the tissue is rendered electrically conductive by treatment in a size press with a slurry containing 5-15 by weight of conductive carbon particles so that the said tissue has a resistance of 5,000 to 100,000 ohms as measured using electrodes of 1 sq. cm., 1 cm. apart, in contact with the said tissue and under a load of 2 kg.

References Cited UNITED STATES PATENTS DAVID KLEIN, Primary Examiner. ROBERT E. PULFREY, Examiner.

J. A. BELL, Assistant Examiner. 0 

